1. Field of the Invention
Embodiments of the invention relate to the field of ion sources used in ion implantation systems. More particularly, the present invention relates to a method for monitoring and evaluating ion source operation.
2. Discussion of Related Art
Ion implantation forms an integral part of modern device fabrication, including such devices as semiconductor integrated circuits and solar cells. In order to be cost effective, ion implanters must be capable of very high throughput and very long and stable operation. A vital component of each ion implanter is the ion source, of which there are several known configurations. For example, an ion source may be a Bernas type source or an Indirectly-Heated Cathode (IHC) ion source. A Bernas type source contains a cathode that includes a hot filament that is located within the region in which a plasma is generated. The requirement of immersion of a hot filament within the plasma can lead to undesirably rapid wear and failure of the relatively fragile filament.
The use of an IHC ion source provides an improved configuration over the Bernas source by placing a bulkier sacrificial component, that is, the cathode, between the filament and the plasma. In a typical IHC source, a cathode body comprises a hollow cylindrical shape that encloses the filament. The function of the filament (often referred to as a heater) is to heat the cathode. In particular, the filament is heated to thermionic emission temperatures by application of a DC current that passes through the filament. A bias voltage is applied between the filament and cathode causing the hot filament to emit electrons. The electrons emitted by the filament are accelerated by the bias voltage and heat the cathode by electron bombardment to temperatures in which thermionic emission occurs. A voltage (the arc voltage) is applied between the cathode and an arc chamber causing the hot cathode to emit electrons. Electrons emitted from the outer surface of the cathode initiate a plasma discharge from which ions can be extracted to form a beam that is used for ion implantation.
An IHC source is configured so that the outer surface of the cathode is immersed in the plasma, while the filament is enclosed and protected from gaseous species in the plasma. Because the bulkier cathode cylinder absorbs wear from the plasma while the enclosed filament does not, filament life is substantially increased leading to an overall increase in the useable life of IHC ion sources. Eventually, a portion of the wall of the IHC cylinder may be eroded away so as to create a hole (puncture) that exposes the filament to reactive gases, leading rapidly to source failure.
Although source failure in an IHC source may result when the filament fails, this knowledge may provide little insight into the IHC ion source lifetime. This is because such IHC ion sources operate for the vast majority of the time without the filament being exposed to reactive gases. Thus, although IHC ion sources enjoy improved life, during operation there may be a large uncertainty as to the actual useable life remaining for such sources. This may lead to unwanted failures of an ion implanter during operation if the IHC source is operated for too long a period or, alternatively, may require scheduling of PM of the ion source with unnecessary frequency in order to avoid such failures. Each of these cases may add significantly to the cost of device fabrication and may otherwise compromise implanter throughput. Accordingly, improved methods and systems for operating IHC ion sources are desirable.